From talking with people, it seems they feel put off by forums and especially long-running threads because they worry anything they have to contribute will already have been posted before, and they don’t know if it’s ok to join an ongoing conversation. They don’t want to annoy people or look silly. In other words, they don’t feel encouraged to post spammy newbie crap, and they’re aware they’re posting to other people instead of to an algorithmic void. Too bad they don’t see that as an advantage instead of a barrier.
e: btw, no matter what tech you use, you will find it very difficult to create a community by encouraging feed-style posting (where you ‘express yourself’ in a wide broadcast to nobody in particular). This drives high-volume, low-quality engagement (which is why fb, twitter, tumblr, instagram etc use it), not high-quality human interaction.